In the case of BGA (Ball Grid Array) or FBGA (Fine Ball Grid Array) packages, problems occur with respect to module reliability, in particular under exposure to changing temperatures. The reason for this can be seen in the different materials used and the resultant different coefficients of expansion, which, although reduced by appropriate material selection, cannot be eliminated.
In addition, the materials used, such as the molding compound, the substrate, the adhesives, etc., absorb water vapor from the surroundings, which depends in part on the ambient conditions and also on the storage times. This causes thermal stresses and stresses induced by the water vapor to occur between the individual components (chip, substrate, molding compound, solder balls). It is possible, that the forces acting on individual interfaces between the materials will reach critical values, which may lead to crack formation and partial or complete delamination (e.g., popcorning) of the package.
In detail, the package contains a chip, for example with at least one central row of bonding pads, the chip being attached on a substrate by means of an adhesive or else a tape (adhesive strip). The substrate, for example a single-layer or multi-layer glass fiber laminate, also referred to as a PCB (Printed Circuit Board), is provided on the side facing away from the chip with solder balls, which are mounted on contacts on the substrate. These contacts are electrically connected by means of interconnects to bonding islands, which are arranged laterally next to a bonding channel in the substrate. The electrical connection of the bonding pads on the chip to the bonding islands on the substrate takes place by wire bridges, which are drawn through the bonding channel. This bonding channel is sealed with a sealing compound after the electrical connections have been established. Furthermore, the chip side is enclosed by a molding compound, which also partly covers the substrate in order to protect the back side and the sensitive chip edges.
Substrate-based BGA packages of this type are usually constructed in such a way that the adhesive area provided for the chip mounting is aligned in a way corresponding to the chip size, in order to ensure secure attachment of the chip on the substrate. In this case, there are different versions with a slight adhesive set-back or projection with respect to the chip.
Particularly disadvantageous in the case of these substrate-based packages is the fact that the adhesive in particular (D/A material) and also the substrate can absorb a relatively large amount of moisture, with the result that the finished package only has a limited reliability, and at present it is usually only possible to achieve MSL3, that is Moisture Sensitivity Level 3. This means that, before soldering with a lead-free solder, the package first has to be heat-treated for a longer time at temperatures around 120° C. in order first to remove the moisture present in the package.
The aim is therefore to achieve Moisture Sensitivity Level 1, at which the package can be immediately soldered right away, without a so-called popcorn effect and resultant detachment of the molding compound and/or the chip from the substrate having to be feared.
In order to achieve favorable conditions here, a composition for an adhesive film that has a particularly high heat resistance and high resistance to moisture absorption is described in U.S. Patent Application Publication 2002/0159773 A1.
Furthermore, Japanese Patent Publications JP 243180/1985 and JP 138680/1986 disclose a printed circuit board material with improved vapor resistance, in which acrylic and epoxy adhesives and also urethane adhesive are used together with an inorganic filler.
However, these measures are not adequate to achieve MSL1. This is caused by the continuing high proportion of D/A material within the package, which can absorb a great amount of water in comparison with the molding compound. The consequence is the so-called popcorn effect, i.e., significant delamination after the preconditioning (storage in humid conditions and simulation of soldering).
The aforementioned problem is countered by a solution according to German Patent Publication DE 101 33 361 C2. In this document, it is proposed that the chip should not be adhesively attached to the substrate over its full surface area but that instead adhesive webs, adhesive strips or adhesive spots should be applied to the carrier substrate to form an adhesive layer, the adhesive consisting of a thermosetting adhesive. In this way, the chip is mounted on the carrier substrate with an intermediate space between the chip and the carrier substrate. However, an adhesive of this type is, by its nature, not yet mechanically stable. Solidifying of the adhesive spots, and with it mechanical stability, is only achieved by introducing a molding compound into the intermediate space between the chip and the carrier substrate and subsequent heating of this molding compound to a temperature which lies above the setting temperature of the adhesive. This does in fact reduce the use of adhesive under the chip; however, there are problems with coplanarity between the chip surface and the carrier substrate. This is so because, until the adhesive layer has set, it is still mechanically movable. If a displacement of the surface of the chip then occurs as a result of filling with the molding compound or other mechanical influences, this surface is no longer planar in relation to the carrier substrate. This is of considerable disadvantage for the subsequent processing processes. In particular, coplanarity is required for the construction of stacks, i.e., a stack of semiconductor chips.
In a similar way, the application of a layer of elastomer material to a substrate is disclosed in PCT Publication WO 01/09939 A1. This layer is pressed onto the substrate in the form of a film. After that, the semiconductor chip is pressed onto the film. Finally, the structure created by it is exposed to a relatively high temperature, to ensure that the semiconductor chip is firmly connected to the substrate. Here, on the one hand, it is likewise not ensured that surface displacements, and with them a risk to coplanarity, will not occur nevertheless when the chip is pressed onto the film. On the other hand, it is always necessary that the processes of applying an adhesive film and positioning a semiconductor chip on the carrier substrate are in close proximity in space and time. This virtually rules out fabrication in the wafer assembly, for instance.